The invention relates to an electric incandescent lamp with a tubular quartz glass lamp vessel having an axis, in which lamp vessel an incandescent body is axially arranged and is laterally surrounded by quartz glass with a red dope. The invention also relates to a quartz glass tube with a red dope suitable for this electric incandescent lamp.
Such an incandescent lamp is known from U.S. Pat. No. 3,258,631.
The known lamp has a lamp vessel which, laterally of the incandescent body, comprises a dope which colors the lamp vessel red, and which is colorless around the portions beyond the incandescent body.
The manufacture of the lamp vessel starts with a tube of porous glass of high SiO.sub.2 content which is impregnated between its ends at the outer surface with a solution of salts, for example, of iron, nickel and aluminium salts, and which is thermally treated in order to convert the salts into oxides and eliminate the pores. The manufacture of the glass tubing for the lamp vessel is very elaborate and therefore expensive.
GB 2,176,587-A discloses an electric incandescent lamp whose incandescent body is laterally surrounded by quartz glass with a red dope in that the lamp vessel is accommodated in a tube of that glass. In this lamp, the tube is coupled to the lamp by being enclosed between the lamp caps of the lamp so as to form a unit. Alternatively, the tube may be a separate body. It is a disadvantage of lamps having an enveloping tube that the lamp vessel can only be linear because bending of concentric tubes and providing a bent tube around another bent tube are either not possible at all or not possible on an industrial scale.
A red quartz glass tube is clamped around the lamp vessel by means of resilient members in other commercially available lamps. Alternatively, a red quartz glass tube may be loosely arranged around the lamp vessel, not as a body coupled to the lamp.
Lamps of the kind described may be used for heating purposes, for example, for room heating, preparation of food such as cooking, or as a heat source in industrial processes such as drying or curing of ink or paint. It is the purpose of the red quartz glass to prevent that the light generated by the lamp is radiated with an unpleasant intensity.
GB 1,036,726 discloses glass compositions of quartz glass with a dope of at least a compound of one of the metals having atomic numbers 22 to 29, 60, 62, 68, 74 and 92, with the possible addition of an aluminium compound. The composition is manufactured from the components by fusion at 1600.degree.-2200.degree. C., in air if purified sand is the basic material, in a vacuum of less than 666 Pa if quartz powder is the basic material.